Automobile air conditioning systems are generally designed to achieve a predetermined air conditioning performance at a predetermined air conditioning load when the automobile is driven at an average range of speeds. But since compressors of automobile air conditioning systems are generally driven by automobile engines through an electromagnetic clutch, when an automobile engine is idling or being driven at low speeds, the rotational speed of a compressor driven by that engine is correspondingly low. Therefore, the performance of such an air conditioning system is adversely affected. On the other hand, when a vehicle is driven at high speeds, the rotational speed of the compressors is too high for efficient performance. In order to overcome these problems, electromagnetic clutches have been used to control the operation of compressors by intermittently stopping and starting the compressors.
However, there are many problems associated with intermittent stopping and starting of a compressor which is known as continuous clutch cycling. For example, when an engine is driven at high speed and the capacity of the air conditioning system is large, it is necessary for the electromagnetic clutch to be turned on and off frequently causing a "shock" each time. On the other hand, at low speed or when the vehicle engine is idling, the compressor may not be sufficiently driven to maintain a desired temperature in the automobile.
In order to solve the above-mentioned problems, a control system has been employed to control the capacity of a compressor in accordance with a detected temperature at the outlet side of the evaporator as disclosed in published Laid-open Japanese Patent Application No. 58-30. In such a system, the performance of the air conditioning system itself is not directly detected. For example, even though the temperature in the compartment of the automobile may be high, the capacity of the air conditioning system may be reduced when the temperature at the outlet side of the evaporator becomes lower than a predetermined temperature. Thus, the capacity of the system is insufficient to cool a compartment of the automobile as desired. In addition, when the automobile is being operated, the capacity of the air conditioning system is changed frequently, thereby placing great stress and strain on the air conditioning system.
In order to solve these and additional problems, a system which controls the capacity of the compressor by detecting the temperature of the outlet and inlet side of an evaporator and comparing the detected temperatures with predetermined temperatures is disclosed in U.S. Pat. No. 4,633,675. In this patent, sensors which are disposed at each of the described locations detect the air temperatures at respective sides of the evaporator. However, for example, when the sensors detect a decrease of temperature, even though the temperatures decrease to a predetermined temperature, sometimes the sensors do not cause a desired predetermined action to occur. Actually, the sensors cause a predetermined action after the temperatures decrease below the predetermined temperature, thereby not being able to timely control the temperature of the air. That is to say, the sensors do not cause an output signal to change the capacity of the compressor even though the temperature reaches a predetermined temperature. This can be caused by the sensitivity of the sensor not being high enough. The result is that the air conditioning capacity cannot be controlled immediately in response to the air conditioning load.